The present invention relates to a device for laying a pipeline on a seabed, comprising a vessel for laying a pipeline, which is fitted with a stinger for guiding a pipe string descending from the vessel and with clamping means for clamping said pipe string to the vessel, and supporting means for supporting the part of the pipe string that extends between the surface and the seabed. The invention furthermore relates to a method for laying a pipeline on a seabed.
So far, the so-called S-laying method has generally been used for laying pipelines. The pipeline is thereby pulled up from the seabed, via a so-called stinger, onto a horizontal deck of a pipe laying vessel in an S-shape as it were. Present on said deck is the welding line. The longer said welding line, the more work stations it will comprise. An increase of the number of work stations leads to an increase of the production rate of the pipeline that is to be laid.
In the case of deeper water, for example in the order of 2000 meters, the pipeline must be bent more in vertical direction by the stinger. If this is not done, the length of the pipeline between the pipe laying vessel and the seabed will become enormous, so that very large forces will be required in order to keep the pipe string intact. If more vertical bending is required, the length of the stinger needs to be increased. Especially in the case of pipes having a large diameter this will lead to a structure which is difficult to handle. Also in the case of a pipe string which leaves the pipe laying vessel more in vertical direction, the weight of the part of the pipe string that extends between the vessel and the seabed will be quite large, albeit much smaller than in the former case. This transmission of weight and strong bending of the pipe string via a long stinger structure is not an easy job.
Consequently, the J-laying method is frequently used, wherein the welding line extends nearly vertically with respect to the pipe laying vessel. In such a case the pipe string need not be bent via a stinger, therefore. A major drawback of this system, however, is the fact that the number of available work stations is limited and that it is not possible to achieve the high production rate that can be achieved with the S-laying method.
The object of the present invention is to provide a device and a method for laying a pipeline on a seabed wherein the advantages of the two above-described methods are combined.
According to the present invention, the device for laying a pipeline on a seabed is to that end characterized in that said supporting means comprise a cantilever structure which is fixedly connected to the vessel and engaging means that mate therewith, which means engage the pipe string so as to transmit axial stresses from the pipe string to the vessel via the cantilever structure.
When pipes are being laid in deep water, the supporting means transmit the mainly vertical pulling force of the pipe string, or the weight of the part of the pipe string that extends between the vessel and the seabed, to the vessel. As a result, the forces that are exerted on the stinger remain small and the stinger construction that is used for laying pipes in shallow water can also be used for laying pipes in deep water. In addition to this the advantage of a high production rate of the S-laying method is retained when using the device according to the present invention. Since the same vessel can be used for shallow water as well as for deep water, a transition from shallow water to deep water will not lead to delays caused by adaptations or changes to other systems that may be required.
In addition to that, the invention relates to a method for laying a pipeline on a seabed wherein a pipe string is bent over a stinger from a pipeline-laying vessel and lowered to the seabed, wherein the pipe string is clamped to the vessel for welding respective lengths of pipeline to the end of the pipe string, and wherein the part of the pipe string that extends between the surface and the seabed is supportedied by supporting means that are fixedly connected to the vessel so as to transmit axial stresses from the pipe string to the vessel via said supporting means.